Introduction to the Motor Effect
After the successful discovery of the relationship between electric current and magnetic field, there comes its important application called the motor effect. The mechanism of producing a mechanical motion by the interaction of electrical energy and magnetism is seen in the motor effect.
It explains how electricity and magnetism act on each other and produce a motion. Simply, they produce a force on each other when placed in each other’s field. Thus, a conductor producing a magnetic field also experiences another magnetic force when kept inside the field of stronger magnets, which is the motor effect. In the same way as arranging in a proper way, it creates continuous rotation.
This theory is the working principle of most of the modern electrical machines like the Electric motors, fans, pumps, loudspeakers etc.
Historical Background and Discovery
Before the discovery of magnetic fields, every electrical device was simply based on the electrostatic force. On the discovery of magnetic fields in 1820 by Orsted, various significant discoveries were made for example, Biot-Savart law, Ampere’s law etc. The combined effect of electricity and magnetism was also studied by Michale Faraday, called the Faraday law. Thus, the force acted by current and the magnetic field on each other was also studied by Ampere. After Ampere’s work, Faraday in 1821 successfully demonstrated the motor effect.
On September 3, 1821, Michael Faraday delivered the first demonstration of the effect using a rotational motion in the Royal Institution’s basement. Faraday brought a free-hanging and submerged it in a pool of mercury and attached a permanent magnet (PM) to the pool. When a current passed through the wire, it rotated around the magnet. This clearly showed that the current created a strong, circular magnetic field around the wire. This work of Faraday was later published in the Quarterly Journal of Science.
The result set the way for the invention of practical AC and DC motors. Gradually, scientists and engineers were able to create machines for converting electricity into motion.
Basic Principle of Motor Effect
The motor effect is defined as the effect of force produced due to a magnetic field on a current-carrying conductor placed inside it. The moving charges interact with the field and produce a magnetic force. This is the basic principle of the motor effect.
As explained by Orsted, the current carrying conductor already has its own magnetic field around it, in its opposite direction. This field around the conductor again comes in contact with the external magnetic field, when the conductor is placed inside the field of a permanent magnet. These fields produce force on the conductor. The direction of the force depends on both the direction of the current and the orientation of the magnetic field. The forces repeatedly act on the conductor and produce a continuous rotational motion on the motors. Thus, this has become a core working mechanism for several electric systems.
Force on a Conductor inside a Magnetic Field
When a wire carrying a current is placed in a magnetic field, the moving charges in the wire experience a force. This force is the Lorentz force. It acts perpendicular to both its velocity and the magnetic field.
For the whole wire, the total force depends on:
- The current (I) flowing through the wire.
- The length (L) of the conductor in the field.
- The magnetic field strength (B).
- The angle (θ) between the current and the magnetic field.
Hence, this force is expressed as, F = BILsinθ [Equation 1].
It is clear from the equation that the force will be maximum if the conductor is perpendicular to the field i.e. θ = 90° and is zero when θ = 0 (conductor parallel to the field).
Thus, conductors in motors experience a torque and the wires move when placed inside a magnet.
Magnetic Flux and Current Direction: Fleming’s Left-Hand Rule
The size and direction of the force depend on both the magnetic flux density and the direction and orientation of the current.
- More field lines crossing the conductor means stronger magnetic fields and produce more interaction with the current. This produces greater force.
- When the current direction is altered, the force direction changes as well. This explains that alternating current can cause changing motion.
- If the wire is parallel to the magnetic field, no force acts on it (since sinθ = 0). The maximum force occurs when the wire is perpendicular to the magnetic field (sinθ = 1).
This relationship shows the importance of correct positioning of conductors to gain maximum output by motors.
The direction of the force on a current-carrying conductor in a magnetic field is easily determined by Fleming’s Left-Hand Rule i.e. on stretching the thumb, fore finger and middle finger at 90° to each other, the fore finger gives the direction of the magnetic field (B), the middle finger points in the direction of the current and the thumb shows the direction of force.
Motor Effect in Uniform vs Non-Uniform Fields
The behavior of the motor effect differs according to the type of magnetic field:
- Uniform Magnetic Fields: When a current-carrying conductor is placed in a uniform field, all of its parts should feel the same force and the field is called uniform magnetic field. A straight conductor has the same force along its whole length. In motors, a uniform field (created by permanent magnets) provides a smooth and uniform torque on the coil. For example, a simple DC motor, where the coil rotates continuously.
- Non-Uniform Magnetic Fields: A non-uniform field means that the magnetic field strength varies over each portion of the wire. Since different parts experience different forces, a net torque or twisting effect is produced in the conductor even if it is not in the proper position. A slightly non-uniform field is able to start rotation in motors since the torque is unequal on both sides of the loop. For example, cathode ray tubes, magnetic bottles etc.
AC and DC Motors
Motors work with both Alternating Current and the Direct Current. The configuration of the current flow determines the type of the motor.
- AC Motor
An AC motor operates with alternating current (AC). The current here is oscillating or periodic, which changes the coil direction automatically. Thus, no commutator is needed. Torque is produced by the interaction of the rotating magnetic field created by AC supply in stator windings and the rotor.
The types of AC motors are the Induction motors and the Synchronous motors. An induction motor doesn’t require an electrical connection to the rotor. The rotor might be wound or squirrel-cage type. It is also called an asynchronous motor.
Three-phase squirrel-cage induction motors are mostly used in industries as they are self-starting and cost-effective. Single-phase induction motors are widely used for smaller loads and stationary generators.
In a synchronous motor state, the shaft rotation matches with the frequency of the supply current, and the rotation period is identical to an integer number of AC cycles.
To produce current in the rotor, it must rotate at a slightly lower frequency than the AC alternations. Small synchronous motors are used in timing applications like synchronous clocks, timers, analog electric clocks, and similar devices.
In typical industrial sizes, the synchronous motor provides an efficient means of transferring AC energy to work, and it can operate at unity power factor, enabling power factor correction.
- DC Motor
A DC motor works when direct current (DC) flows through a coil placed in a magnetic field. The coil experiences torque that creates rotation in the motor. A commutator is used to reverse the current direction in each half turn to keep the coil rotating in the same direction. They are the traditional motors for power generation. DC motors have components like Stator or electromagnets to provide magnetic field, Armature/rotor and a Commutator to switch the current direction. They have been used in toys, electric vehicles, fans, cranes, and battery-powered devices.
Classroom Experiments of the Motor Effect
The motor effect can be easily demonstrated in the classroom.
- Straight Wire in a Magnetic Field: Place a current-carrying wire between the poles of a U-shaped magnet. When current flows, the wire shows deflection.
- A battery connected with wire in a Field: Place a battery over the magnet and bend the wire touching battery and the magnet. Let the current pass through it. This creates a motion in wire which demonstrates the principle of motor effect.
These demonstrations can visualize simply, how an electrical energy can be changed into kinetic energy or mechanical motion.
Applications of Motor Effect
Our basic household devices to complex power generating systems utilize motor effects for their operation. Some of the significant applications are given below:
- Electric Motors
It is the most popular and important application of motor effect. The coils carrying current are placed in a magnetic field. This creates a torque that makes the the rotor rotate. Many familiar appliances like electric fans, mixers, electric vehicles, washing machines, and industrial machinery are driven by this effect.
- Loudspeakers
A magnetic field is surrounded upon a current-carrying coil. As AC flows through the coil, it starts oscillating back and forth, creating vibration on the speaker cone. This results in sound production. The same principle lies on the headphones.
- Galvanometers and Moving-Coil Instruments
As described by motor effect, a coil carrying current is placed in a magnetic field which produces a torque. The torque moves the needle of the measuring devices like Galvanometers and Ammeters used to measure current.
- Railguns and Linear Accelerators
A projectile is projected forward with very high speed using railguns. They have two parallel conducting rails connected to a power source. A conducting armature is placed between them. A large current is passed through the rails and a projectile to get a very strong magnetic field around the rails. Due to the motor effect, the projectile experiences a force. This force propels the projectile forward.
In linear accelerators, the motor effect provides the magnetic control and focusing required to maintain charged particles on trajectory. Particle beams would show rapid fluctuations without the motor effect which would disqualify the accelerator.
Torque on a Current Loop
When a loop of wire is placed in a magnetic field, the forces are produced on opposite sides in opposite directions. This produces a turning effect called torque. This keeps the motor running, while fixed on the arm. The torque increases with the number of turns in the coil because the force applies on several conductors at the same time. This technique enables motors to create strong and rapid rotations in certain machineries.
Advantages and Limitations of Motor Effect Devices
Advantages:
- Provides a direct conversion of electrical energy into motion, smoothly.
- Can work with both AC and DC supply.
- Can be equally used for simple household appliances to huge industrial motors.
- Reliable and widely used concept modern technology.
Limitations:
- Efficiency can be reduced by friction, heat, and flux leakage.
- Motors require materials like copper and magnetic cores, which can be costly.
- Very strong fields may cause saturation of the magnetic core.
- Regular maintenance may be needed for the components.
Conclusion
Motor effect has driven the technology to an outstanding level. It has paved roads for modern electricity and engineering. Starting from Orsted, it has become the most widely used and reliable concept of electromagnetism. The world has become totally dependent on electricity and magnetism in contemporary days. The flexibility of motor effect to work on both AC and DC has made it an even more suitable and efficient concept to convert electrical energy into kinetic energy.
In addition, we can see how a simple concept of electromagnetism has controlled our lives from cooking, laundry to industrial purpose. Its simple demonstrations have made it even easier to understand and make it understand to basic levels. Slight limitations can be easily overcome on proper handling and frequent checkups. This is a burning example of how a simple discovery can influence the era with its outstanding implications.
References
Toliyat, H. A., & Kliman, G. B. (2018). Handbook of electric motors. CRC press.
Kamberaj, H. (2022). Electromagnetism. Springer.
Fernow, R. C. (2017). Principles of magnetostatics (p. 300). Cambridge University Press.
Ida, N. (2020). Faraday’s law and induction. In Engineering Electromagnetics (pp. 505-549). Cham: Springer International Publishing.
https://en.wikipedia.org/wiki/Electric_motor
https://byjus.com/physics/the-electric-motor/
